1. Field
This invention relates generally to hot functional testing of nuclear power plants and, more particularly, to the development of protective oxide films on primary system material during hot functional testing.
2. Related Art
Nuclear power plants generally include a primary system, which includes a reactor vessel, steam generators, reactor coolant pumps, pressurizer and connecting piping. A reactor coolant loop includes a reactor coolant pump, a steam generator and piping that connects these components to the reactor vessel. Nuclear power plants can have two, three or four reactor coolant loops. The nuclear fuel is contained in the reactor core, which is housed in the reactor vessel. A function of the primary system is to transfer heat from the nuclear fuel to the steam generator.
In order for the primary system to perform its function, coolant water circulates through the primary system. System materials and surfaces that are contacted by the coolant water will undergo general corrosion. The development of passive, e.g., protective, oxide films on primary side surfaces, e.g., metal surfaces, is important for controlling corrosion of the materials and component, and for controlling corrosion product release during power operation. As metal corrodes, metal oxide is generated. A portion of the metal oxide forms an adherent layer on the primary side surfaces and a remainder of the metal oxide is released into the coolant water. As the adherent oxide layer develops, it becomes increasingly passive, e.g., protective, and slows the corrosion reaction with the metal surfaces, and eventually the corrosion reaction is reduced to a low, steady state value.
The primary system in Pressurized Water Reactors (PWRs) is the Reactor Coolant System (RCS). In PWRs, corrosion occurs and deposits form when structural materials in the RCS are exposed to high temperature reactor coolant during plant operation. These corrosion products are subsequently released into the reactor coolant and can deposit on the fuel in the reactor core. Historically, significant effort has been expended in the selection of corrosion resistant materials, as well as development of chemistry control additives and plant operating practices to minimize corrosion formation and deposition. During normal plant operation, chemistry control is used to develop passive films on primary system materials and surfaces to control the corrosion rate and corrosion release rate during power operation, in order to minimize the deposition of corrosion products on the core as “crud” and subsequent activation. This is necessary to minimize the risk for crud-related fuel performance issues and plant dose rates.
New nuclear plants undergo hot functional testing (HFT) prior to loading fuel into the nuclear core (pre-core) to demonstrate operability of the plant. During pre-core HFT, an initial (primer) oxide layer develops on surfaces in the primary system, e.g., RCS, upon initial exposure to high temperature coolant water. Following pre-core HFT, during subsequent commercial operation of the nuclear plant, this primer oxide layer impacts the inventory of corrosion products available to deposit on the core and, thereby, impacts fuel crud risks and plant dose rates. Thus, it is believed that the primer oxide layer affects the corrosion resistance for the life of the nuclear power plant.
PWRs that started-up in the 1970's and 1980's had performed HFT using only lithium hydroxide for chemistry control of the primary system, in order to maintain an alkaline pH. Since that time, plants have performed HFT using dissolved hydrogen as well, to develop more protective oxides on RCS surfaces. Also, at least one plant has used boric acid and hydrogen peroxide during HFT to simulate the chemistry conditions during a refueling shutdown, which results in the dissolution of corrosion products such that they can be easily removed from the system by ion exchange.
In the commercial operation of nuclear plants, experience has shown that there are benefits to be derived from zinc injection. For example, it has been found that as zinc is incorporated into existing corrosion films on primary system surfaces, e.g., RCS surfaces, the oxides become more stable and protective, inhibiting both general and localized corrosion. The vast majority of PWRs performing zinc injection are mature plants that started this process after a significant period of operation, e.g., 15 to 20 years, with normal primary water chemistry. The zinc is only injected at normal operating temperatures during power operation. Thus, in these mature plants, the existing oxide films have been formed and established on metal surfaces as a result of continued plant operation. Nickel and cobalt atoms are present in the existing oxide films, and zinc injection is used to restructure the existing oxide films. The restructuring process can continue over many fuel cycles until the existing oxide films are restructured with high concentrations of zinc and chromium present near the metal oxide interface. Zinc injection in operating nuclear plants can cause the additional release of particulate or dissolved corrosion products into the coolant as the zinc atoms are incorporated into the existing oxide films and they replace or displace other atoms, e.g., nickel and cobalt atoms. The release of these additional corrosion products into the coolant can increase the concentration of corrosion products circulating in the primary system, which can increase the amount of material available to deposit on the fuel and thereby, potentially increase the risk for fuel performance issues.
This additional risk limits the concentration of zinc that can be used during power operation, especially for PWRs with higher subcooled nuclear boiling. The subcooled boiling process provides a mechanism for circulating corrosion products to concentrate and deposit at the cladding surface of the nuclear fuel element in the reactor core. Crud deposition occurs in areas of the reactor core undergoing subcooled boiling to a much greater extent than on non-boiling surfaces. Once porous crud deposits are present, the boiling process also provides a mechanism to concentrate any contaminants in the coolant within the crud layer. Increased crud deposition may lead to increased risk of Crud Induced Power Shift (CIPS), also known as Axial Offset Anomaly (AOA). The risk of Crud Induced Localized Corrosion (CILC) may also be increased. CIPS occurs when crud deposits become sufficiently extensive and sub-cooled boiling rates are sufficiently high to result in precipitation of significant amounts of lithium-boron compounds within the crud layer. This results in a shift in axial power distribution away from the boron deposits. Locally, thick crud deposits can also reduce heat transfer and increase fuel cladding temperatures, which can lead to CILC.
Another concern associated with adding zinc to coolant water in an operating nuclear plant is the potential for zinc oxide or zinc silicate to deposit within the crud of the fuel cladding. Such deposition may decrease the transfer through porous crud and thereby, potentially increase fuel cladding corrosion. The likelihood of this scenario can increase if the boiling concentration process within the crud causes the zinc concentration to exceed the solubility limit of either zinc oxide or zinc silicate. To reduce or preclude these fuel risks in an operating nuclear power plant, the level of zinc in the coolant is monitored and controlled, and typically limited to 40 ppb or less during power operation.
It has been found that pre-core HFT provides a unique opportunity to initiate the development of protective oxide films on primary system surfaces and to remove releasable corrosion products, which can prevent deposition and neutron activation during power operation. Further, since the nuclear plant has not been operating prior to pre-core HFT, no existing oxide films have been formed and therefore, the initial protective oxide films formed on the RCS surfaces during pre-core HFT can include zinc incorporated therein without the need to restructure existing films. Tomari 3 in Japan was the first PWR in the world to inject zinc during hot functional testing, but the levels used were similar to that during power operation, i.e., 3 to 7 ppb. Since fuel is not loaded during pre-core HFT, the amount of zinc injected into the coolant water can be increased without the risk of fuel-related concerns and issues. Thus, there is a need in the art to improve the control of reactor coolant chemistry during pre-core HFT as a preconditioning process to passivate the RCS surfaces prior to normal power operation of a nuclear power plant, in order to control corrosion and optimize the long-term integrity and performance of plant systems in the nuclear power plant. This will also significantly reduce the inventory of corrosion products available to deposit on the fuel and become activated during normal power operation, thereby improving fuel performance and minimizing plant dose rates.